Python for loop展望未来

您可以使用此配方获得任何可以预取下一个项目的项目:

from itertools import tee, islice, izip_longest
def get_next(some_iterable, window=1):
    items, nexts = tee(some_iterable, 2)
    nexts = islice(nexts, window, None)
    return izip_longest(items, nexts)

示例用法:

for line, next_line in get_next(myfile):
    if next_line and next_line.startswith("0"):
        ... do stuff

代码允许您通过 window 如果要向前看2行或更多行,请将参数设置为更大的值。

你可以有一个 prev_line 存储前一行并在仅给定条件时读取该行时进行处理的位置。

类似于:

prev_line = None
for line in file:
    if prev_line is not None and the start of the next line == "0":
        perform pre-processing on prev_line
        ...
    continue with normal processing
    ...
    prev_line = line

如果需要,您可能需要对最后一行进行额外的处理,这取决于您的逻辑。

按照nosklo的回答,我倾向于使用以下模式:

功能 pairwise 从优秀的 itertools recipes 非常适合:

from itertools import tee

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    next(b, None)
    return izip(a, b)

在代码中使用它可以让我们:

for line, next_line in pairwise(file):
    if next_line.startswith("0"):
        pass #perform pre-processing
        #...
    pass #continue with normal processing

通常,对于这种类型的处理(iterable中的lookahead),我倾向于使用 window function . 两厢是2号窗户的特例。

你只需要缓冲一行。

for line in file:
  if (prevLine is not None):
    //test line as look ahead and then act on prevLine
  prevLine = line

这也应该管用。我总是喜欢打电话 next 过度设置 something = None 第一轮。

prev_line = next(the_file)
for current_line in the_file:
    if current_line.startswith('0'):
        do_stuff( prev_line )
    # continue with normal processing
    # ...
    prev_line = current_line

more_itertools 有几个 lookahead tools . 在这里,我们将演示一些工具和处理文件行的抽象函数。鉴于:

f = """\
A
B
C
0
D\
"""
lines = f.splitlines()

代码

import more_itertools as mit


def iter_lookahead(iterable, pred):
    # Option 1
    p = mit.peekable(iterable)
    try:
        while True:
            line = next(p)
            next_line = p.peek()
            if pred(next_line):
                # Do something
                pass
            else:
                print(line)
    except StopIteration:
        return


pred = lambda x: x.startswith("0")
iter_lookahead(lines, pred)

输出

A
B
0

以下是使用同一个库的其他选项,包括 pairwise 和 windowed @Muhammad Alkarouri提到的工具。

# Option 2
for line, next_line in mit.pairwise(lines):
    if pred(next_line):            
        # Do something
        pass
    else:
        print(line)

# Option 3
for line, next_line in mit.windowed(lines, 2):
    if pred(next_line):            
        # Do something
        pass
    else:
        print(line)

后一个选项可以独立运行或替换先前函数中的逻辑。

我不是Python专家,但我想您需要为此使用两个循环。for循环的第一次运行应该构建一个索引列表,需要对其执行特殊操作。然后在第二次运行时,可以将当前索引与列表进行比较,以确定是否需要执行该特殊操作。